The present invention relates to the electrical polarization of a liquid crystal display screen or LCD screen in order to obtain an image whose readability depends as little as possible on the ambient luminosity.
A liquid crystal cell consists of a thin layer of liquid crystal with a twisted nematic structure or helix nematic structure enclosed between two transparent sheets fitted out with polarizers and subjected to a variable electrical field. When there is no electrical field, and with an appropriate treatment of the internal walls of the plates, the molecules of a liquid crystal get organized in twisted or helical structures with an axis perpendicular to the two transparent planes. These structures have the property of making the polarization of light rotate whereas in the presence of an electrical field the molecules tend to get aligned in the electrical field and lose their capacity to make the polarization rotate. Thus, when the thickness of the liquid crystal layer between the two transparent plates is such that the polarization of light rotates by 90xc2x0, it is enough to cross the polarizers to have a screen that is transparent in the absence of an electrical field and a screen that is opaque in the presence of an electrical field. By locally bringing into play the value of the electrical field at each point of the surface of an LCD screen, it is then possible to modulate the light by transmission and generate images. This is obtained by the application, by means of amplifiers, of a polarizing voltage between transparent electrodes distributed in a matrix arrangement, facing each other on the two transparent plates of an LCD screen. This polarizing voltage changes between two levels: a low level for the transparent state and a higher level for the opaque state.
To obtain sufficient contrast with an LCD screen under variable conditions of low and high ambient luminosity, it is necessary to have a high dynamic range of polarizing voltage. There is therefore a tendency to increase the dynamic range of voltage for the polarizing of an LCD screen but then two problems arise: firstly, the increase in contrast obtained is not linear as a function of the gray levels, thus prompting defects in the image such as reversals of contrast and, secondly, there is an increase in electrical power consumption.
The present invention is aimed at improving the readability of an LCD screen polarized under varied conditions of ambient luminosity without thereby increasing, to any extent, the dynamic range of the polarizing voltage so as to limit the defects of polarization reversal and obtain savings in electrical power consumption.
An object of the invention is a method for the electrical polarization of an LCD screen as a function of the ambient luminosity, consisting of the measurement of the brightness of the ambient luminosity and of the polarizing of said screen by means of a polarizing voltage changing in a range of variation that is shifted, as a function of the ambient luminosity measured, towards voltages that are higher in terms of absolute value when the ambient luminosity diminishes or towards voltages that are lower in terms of absolute value when the ambient luminosity increases.
Through a shift of this kind in the range of variation of polarizing voltage as a function of the ambient luminosity, there is obtained, firstly, for the control of the opaque state of the screen, a voltage for the polarizing of the screen that is all the higher in terms of absolute value as the ambient luminosity is low, thus favoring the black level to the detriment of the white level or color levels under normal or low ambient luminosity and, secondly, for the control of the transparent state of the screen, a polarizing voltage that is all the lower in terms of absolute value as the ambient luminosity is high, thus favoring the white level or the color levels to the detriment of the black level under high ambient luminosity.
An object of the invention is also a device for the polarizing of an LCD screen comprising a counter-electrode on one face and electrodes distributed in a matrix arrangement on the other face, this device implementing the above-mentioned method. This device is provided with an amplifier that generates a reference polarizing voltage for the counter-electrode and amplifiers generating voltages for the polarizing of the gray levels ranging from the white level to the black level for the electrodes distributed in a matrix arrangement. It furthermore comprises a detector of ambient luminosity, a threshold comparator connected at input to the detector of ambient luminosity and differentiating between two conditions of ambient luminosity, one being a low or normal condition and the other being a high condition, and a memory with two zones that are addressed in alternation by said threshold comparator, this memory storing a distinct value for the reference polarizing voltage of the counter-electrode in each of its zones and being connected in the read mode of its zones to the input of the amplifier generating the reference polarizing voltage for the counter-electrode.
According to a preferred embodiment, the memory also stores, in its two zones, two distinct sets of polarizing voltages corresponding to two different scales of reference gray levels, and is furthermore connected, in the read mode of its two zones, to the input of the amplifiers generating the gray level polarizing voltages for the electrodes distributed in a matrix arrangement.